node architectures for optical packet and burst...
TRANSCRIPT
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Node architectures foroptical packet and burst
switching
Chris Develder,Jan Cheyns, Erik Van Breusegem, Elise Baert,
Ann Ackaert, Mario Pickavet, Piet Demeester
Dept. of Information Technology (INTEC)Ghent University, Belgium
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 2
Outline
• Introduction– OPS and OBS concepts– packet & header format
• Node architecture– functionality– optics and/or electronics
• Switch matrix– alternatives– scalability
• Contention resolution– problem & solution– buffer architectures
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Introduction
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 4
Optical switches
• Optical switching:• direct light from an
input port to an output port• possibly wavelength conversion
• circuit-switching:• continuous bit-stream• pre-established light-paths• set-up: “manually” or dynamic
• packet/burst switching• chunks of bits, encapsulated in packets• packet header determines forwarding• e.g. label switching, GMPLS based f f
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 5
Packet format
• fixed/variable duration:– pro variable = no fragmentation/
reassembly, no padding, less header overhead
– contra = long packets can block many short ones
• slotted/unslotted operation:– pro slotted = easier packet scheduling
(synchronous switching)– contra = cost of synchronisation
components
• OPS = fixed, slotted packets• OBS = variable, unslotted packets
λ1
λ2
unslotted, variable length
λ1
λ2
slotted, variable length
λ1
λ2
slotted, fixed length
padding
single packet
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 6
Header format
–
– out of band: orthogonal channel (e.g. DPSK)
–
– out of band: dedicated wavelength; also multi-wavelength headers have been proposed (see e.g. PS.TuC1)
• position of header:– in band: header and payload are sent sequentially,
separated in time
λ1
λ2
λ3
λ4
λ1
λ2
λ3
λ4
λ1
λ2
λ3
λ4
phase
intensity
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 7
Operation of OPS
• fixed-length packets, slotted operation• header accompanies payload
• contains necessary information to make forwarding decision
• each timeslot:• inspect packets at input ports• decide which packets can be forwarded without collisions
• switch is “memory-less”• no knowledge of packets scheduled in past is necessary
OPSnode
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 8
Operation of OBS (1)
• variable packet lengths, unslotted operation• header is sent Toffset before payload
• contains necessary information to make forwarding decision• functions as one-way reservation (allows timely config. of switch fabric)• offset decreases by header processing time per hop
• on arrival of header:• decide whether burst can be forwarded without collisions• make necessary resource reservations if burst is accepted
• switch needs “memory”:• keep track of reservations made in past
OBSnode
Toffset - δ
Toffset
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 9
Operation of OBS (2)
• Note: WR-OBS = wavelength-routed OBS– two-way reservation
• “header” is sent from source to destination within OBS network• if all goes well, acknowledgement is sent back to source
– this is more like wavelength switching at very short timescales – proposed by P. Bayvel et al. (Univ. College London, UK)
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Node architectures
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 11
Functionality of an OPS/OBS node
• input interface:• header extraction (straightforward if out-of-band)• synchronisation: detect beginning of packet/burst• in OPS: align packets
• switching matrix (see further)• output interface
• e.g. regeneration of optical signal; header re-writing…
synchr.control
switchcontrol
headerrewriting
inputinterface
switching matrix
outputinterface
payload
header
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 12
Combine the best ofelectronics and Optics
• optical packet switching “today”:• header is processed electronically• payload is switched optically
⇒ optics for capacity & switching,electronics for routing & forwarding
• note:• all-optical header processing is “under study” (e.g. PS2001, PThD5)
synchr.control
switchcontrol
headerrewriting
inputinterface
switching matrix
outputinterface
optical processing
electronicprocessing
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 13
Oh-oh-oh*
: optical : electrical• O-O-O:
– optical input interface, optical switch fabric, optical output interface
– payload is switched transparently, without leaving optical domain
– pro = bitrate-transparent– con = still emerging technologies, BER can not be monitored
• O-E-O:– optical inputs, converted to el. for switching, back to optics at
outputs– “opaque”: no more all-optical; but straightforward grooming– pro = well established techno, 3R regen. “for free”– con = no bit-rate transparency, scalability ~ Moore’s law
• OEO-O-OEO– (some) inputs and outputs: electronic 3R regen.– pro = scalability of optical switch fabric with 3R regen.– con = bit more complex than O-O-O
*: G.Bennet, at www.lightreading.com
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Switch fabric
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 15
Overview of dominant architectures
• dominant approaches to optical switch fabrics:• MEMS (=micro-electro-mechanical systems)• broadcast-and-select (e.g. SOA-based)• AWG and tuneable lasers
• MEMS:– principle:
– main components:• tiny mirrors (2D pop-up, or 3D tilting)
– characteristics• low loss, good scalability (=high port counts)• but… too slow for packet switching (ns timescale not feasible)
© Lucent, e.g. OFC2002
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 16
Broadcast-and-select switch
• principle: (e.g. David, http://david.com.dtu.dk; ECOC’01)
• main components:– splitters– selectors
• characteristics:– split losses: calls for regeneration– inherent multicast capability
l1lm
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1:(m.n) n:m
lj
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l1
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l1
complete fibre is broadcast to all output ports
first array of SOAs selects 1 input fibre
second array of SOAs is used to select 1 wavelength
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 17
AWG based switch
• principle: (e.g. Stolas, http://www.ist-stolas.org)
• main components:– tuneable wavelength converters– AWG
• characteristics:– passive component, no split losses– multicast quite complex
TWC
TWC
TWC
TWC
TWC
TWC
TWC
TWCAWGAWG
1 2 3 4 5 6 7 8
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1 λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8
λ1
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out-p
ort
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 18
Scalability
• switch dimensions limited by• B&S: split losses• AWG: tuneability range of TWCs
• Possible solution:• multi-stage switches, e.g. Clos-networks
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 19
A three-stage Clos-network (1)
• slotted OPS• re-arrangeable non-blocking• second-stage switches: k ≥ n
• unslotted OBS• fully non-blocking• second-stage switches: k ≥ 2n-1
N/nswitches
kswitches
...
...
...
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3-stage Clos switch
Noutput ports
A
B
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...
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put p
orts
Noutput ports
k x nN/nx
N/n
n x k
k x nN/nx
N/nn x k
n x k
k x nN/nx
N/n
⇒slotted OPS needs only half as much second stage switches
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 20
A three-stage Clos-network (2)
• note: if wavelength conversion is allowed, third switching stage can be eliminated
• e.g. slotted OPS:• F fibres, W wavelengths per fibre• make 1st and 3rd stage per input/output fibre
......
...WxW
WxW
WxW
FxF
FxF
FxF
Fswitches
Wswitches
WFconvertors
WF input ports
WF
output ports
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Contention resolution
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 22
Problem and possible solutions
• Problem:• two or more packets contend for same resource: destined for same
outgoing port at the same time
• Solutions:• deflection routing• wavelength conversion• buffering: optical buffer = Fibre Delay Lines (FDLs)
contention
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 23
What solution to choose?figures © Yao et al., Opticomm’00
netw
ork
thro
ughp
utpa
cket
loss
rate
load (packet arrival rate, pkt/s)
load (packet arrival rate, pkt/s)
• Deflection:• packets are “stored” in network:• increases load, increases delay• only works for low loads
• Wavelength conversion:• no packet storage• allows high network throughput, no
increased delay
• Buffering:• local packet storage at nodes• small delay penalty
⇒ Use combination of wavelength conversion and buffers
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 24
Buffer architectures
feed-forward vs feed-back• feed-forward vs feed-back
• feed-forward: input or output buffering
• feed-back: shared, recirculatingFDLs
• single stage vs multiple stage• multiple stages separated by
switching elements• e.g.: each stage different delay
resolution (“units”, “tens”, “hundreds”…)
• choice of FDL lengths• multiple FDLs: multiples of “unit”,
resolution D• uniform/non-uniform (D,2D,3D or
something else)
single vs multiple stages
D-1
10
...
D-1
10
D-1
10
... ...
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 25
OPS/OBS: fixed vs increasing FDLs
1
B
…1
1
1.E-07
1.E-05
1.E-03
1.E-01
0 8 16 24 32 40 48 56 64nr. buffer ports (B)
ParetoOnOff, incr ParetoOnOff, fix
GeoOnoff, incr GeoOnoff, fix
Poisson, incr Poisson, fix
see COIN.TuD1 for details
1
B
…
…
1
B
• sample results for fixed-length, slotted OPS
• Increasing FDL lengths give far lower PLRs (order of magnitude or more)
• “penalty”: reordering of packets, higher delays
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 26
OBS: choice of granularity
sample results for slotted switch with output buffering, single wavelength per fibre, geometric distr. packet lengths, bernouilliarrivals, 20 FDL lengths
• Optimal granularity D:⇒Non-trivial choice!
• Tradeoff between resolution and buffering capacity
• small D: small gaps, but limited buffer depth
• large D: large buffer depth, but large gaps between packets
• Function of• load• traffic profile (packet size
distribution, burstiness…)
D-1
10
...
pack
et lo
ss p
roba
bilit
y
granularity D
1E+0
1E-2
1E-4
1E-6
0 50 100 150
ρ=0.6
ρ=0.5
ρ=0.4
optimal D depends on load
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 27
OBS: void filling
• Void creation in case of variable packet-lengths (OBS):• FDL buffer offers discrete set of delays
(in contrast to e.g. electronic RAM)• gaps between successive packets: “voids”
⇒Need for intelligent buffer scheduling if we want to improve link throughput (i.e. void filling)
OBS node+ FDL buffer
d D-d
example: we need delay d, but FDL only offers D ⇒ gap of D-d is created;void filling will attempt to insert packet in this gap
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 28
Contention resolution
• wavelength conversion greatly reduces need for buffering
• feed-back architecture allows sharing of FDL resources among all output ports
• when using different delay lengths, this calls for more intelligent buffer scheduling; for variable-length packets (OBS) the issue of void creation arises
• OBS: choice of delay lengths, i.e. the FDL granularity, is non-trivial issue
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Conclusions
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PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 30
Summary
• reviewed OPS/OBS concepts– OBS as possible 1st step– OPS: fully exploit fast switching technologies
• switch architectures:– MEMS: too slow for OPS– broadcast & select: allows multicast, but splitting losses– AWG: passive core component, but no multicast– multi-stage architecture to increase scalability
(OPS fewer switching elements than OBS)
• buffering:– push buffers to network edges– use FDLs to lower PLR in core– non-trivial choice of FDL granularity for OBS– quite complex scheduling for OBS
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That’s all, folks!
… thanks for your attention